Iraqi special forces seized control of state television facilities inside Mosul on Tuesday, as a loose-knit combat coalition allied with Baghdad pressed an offensive to drive Islamic State extremists from the city.

Kurdish and Western journalists embedded with the force said it was meeting stiff resistance inside the city, encountering concrete blast walls and roadside bombs planted by IS fighters to slow the Iraqi advance.

The Iraqi military also said an armored division was approaching southeastern districts, as part of a push to free outlying villages and encircle the city.

Separate coalition units pressing toward Mosul from the north were also reported in control of several key villages.

The long-awaited offensive comes two weeks after the coalition of Iraqi and Kurdish forces — backed by Shiite militia, Sunni Arab tribesmen and U.S.-led airstrikes — began the largest military operation in the country since 2003 to clear the city of IS fighters.

In Washington on Tuesday, the U.S. anti-terror special envoy Brett McGurk voiced optimism about the offensive, telling Radio SAWA it is now simply "a matter of time before Iraqi security forces enter and liberate this great city." However, he also called for patience, cautioning that retaking the city will take time and will unfold in stages.

Displaced people who fled from Islamic State-held territory sit outside a mosque guarded by Iraqi soldiers in Shuwayrah, south of Mosul, Iraq, Nov. 1, 2016.

Meanwhile, in Geneva, United Nations spokeswoman Ravina Shamdasani said IS fighters are continuing their push to forcibly move thousands of civilians closer to Mosul to provide extremists inside the city with human shields.

She also voiced alarm over new reports of Islamic State mass killings, including information from "reliable sources" that 40 former Iraqi soldiers were killed Saturday near Mosul, their bodies then thrown into the Tigris River.

U.N. estimates show up to a million people could become displaced as fighting intensifies in Mosul. Nearly 18,000 people have already fled the city since the operation was launched October 17.

Iraqi special forces soldiers use radio communication as a tank rolls down an alley in the village of Bazwaya, some 8 kilometers from the center of Mosul, Iraq, Oct. 31, 2016.

Separately Tuesday, neighboring Turkey began shifting armor and other vehicles to a key district on the Iraqi border, making what Defense Minister Fikri Isik described as preparations "for all possibilities."

Turkish President Recep Tayyip Erdogan has said he wants to protect ethnic Turks residing in Iraq, Iran and Syria from the presence of Iran-backed Shiite militia participating in the coalition fighting to retake Mosul.

He has also said his government is trying to contain the Kurdish PKK faction, which has battled the Ankara government for decades to establish an autonomous homeland in Turkey's southeast near the borders of Syria and Iraq.

Turkey, the United States and the European Union have designated the PKK as a terrorist organization.